Method and apparatus for diagnosing and remediating language-based learning impairments

ABSTRACT

The present invention provides an improved method and apparatus for the identification and treatment of language perception problems in specific language impaired (SLI) individuals. The invention provides a method and apparatus for screening individuals for SLI and training individuals who suffer from SLI to remediate the effects of the impairment by using the spectral content of interfering sound stimuli and the temporal ordering or direction of the interference between the stimuli.

BACKGROUND OF THE INVENTION

Research suggests that between three and six percent of individuals whoare otherwise unimpaired have extreme difficulties producing andunderstanding spoken language. This disorder is typically labeledspecific language impairment. Persons diagnosed with specific languageimpairment (SLI) often have accompanying reading difficulties, but notall children with reading difficulties have specific languageimpairment.

Current research suggests that many individuals with language learningimpairments have a "temporal processing deficit" that can bedemonstrated by psychoacoustic measurements of several different types.They have special problems in identifying and therefore correctlysequencing rapidly successive acoustic stimuli because those successivestimuli interfere destructively with one another. This abnormal signalreception is believed to be accounted for by abnormally long integrationof acoustic information in the impaired individual's neuronal processingof the sound input stream. On the basis of these findings, procedureshave been developed to assess the "temporal threshold" in impairedindividuals using a stimulus sequencing task in which pairs ofsuccessively presented tonal or vowel-like stimuli were varied in theirdurations and interstimulus intervals. The temporal threshold is thetime separation (interstimulus interval) between two stimuli that arenearby in frequency, and that interstimulus intervals. The temporalthreshold is the time separation (interstimulus interval) between twostimuli that are nearby in frequency, and that are of fixed frequency,intensity and duration, at which they can be accurately sequenced by theindividual. This method can and has been used as a basis for screeningchildren and adults to identify those with acoustic reception-basedlanguage learning problems, and at risk for dyslexia (reading failure).

The abnormal temporal interferences that distinguish the languageimpaired child appear to underlie the "fuzzy" phonetic reception and"phonological awareness" deficits that also plague them. It has beenspecifically demonstrated that language impaired children cannotdistinguish between specific phonemes (for example, the consonant-vowelcontrasts /da/ and /ba/) when the consonant transitions are presented ina fast form, but can identify them if the consonant transition isadequately prolonged in time. This has been interpreted as occurringbecause in its long form, critical consonant features are representedoutside of the domain of destructive interference or integration thatdestructively alters briefer stimuli.

Studies conducted in animal models and experiments conducted in humansubjects performing visual tasks have demonstrated that the correctidentification of rapidly successive inputs can be improved markedlywith training. On the basis of these new experimental findings and thebackground perspective on the acoustic reception problems of languageimpaired individuals described above, a training method was invented toameliorate the "temporal processing deficits" of language impaired anddyslexic individuals, and to facilitate complex acoustic signal andlanguage processing in general. That training approach is the subject ofU.S. Pat. No. 5,813,862, to Merzenich et al., issued Sep. 29, 1998,entitled, Method and Device for Enhancing the Recognition of SpeechAmong Speech-Impaired Individuals, which is expressly incorporatedherein by this reference. The teachings of that patent applicationprovide a highly effective basis for overcoming, through training, theacoustic sound stream interference problems that limit signal receptionin language impaired individuals. With the dramatic "clarification" ofspeech input that results from such training, children have been foundto perform at much higher levels at a wide variety of phoneticreception, speech/language memory, syntactic, semantic, grammaticalreceptive and expressive language tasks. In general, after training,their language skills are elevated to normal or nearly normal "languagequotient" levels.

The particular techniques described in the above-identified patentapplication focus on the difficulty that SLI persons have indiscriminating between a pattern of brief sounds that occur in closetemporal proximity to each other. The adaptive training methods taughtin that patent application ordinarily involve altering the brief sounds,by prolonging them or increasing their level (intensity), and increasingthe interstimulus interval (ISI) between the sounds. These changes inthe sounds and in their temporal separation makes them more intelligibleto the SLI person. The training program typically involves a regimen ofrepeated presentation of the modified sounds in a controlled pattern tothe SLI individual. The individual is asked to identify the shortduration sounds. The ISI between the brief sounds is gradually decreasedas the individual's ability to identify the sounds improves. Theseimprovements in the ability of the individual to discriminate betweenthe sounds is monitored throughout the training program. As theperceptual acuity of the person continues to improve, the ISI isdecreased even further and the repetition of sounds is repeated with thenew ISI. It has been found that, over the course of a training program,an SLI individual's ability to distinguish between brief soundspresented in a rapid sequence may improve significantly. Eventually, asa result of the training, the SLI person may be able to discriminatebetween substantially unmodified brief sounds, such as consonants, whichare temporally separated by an amount typical to a normal speech rate.

While these earlier procedures for the identification and remediation oflanguage perception problems in specific language impaired individualsgenerally have been highly effective, there exists a need to adopt newprocedures consistent with ongoing developments in the understanding ofthe nature and causes of these perceptual problems. For instance, theseearlier treatment methods generally do not consider the potential impactof the ordering of interfering sounds upon the ability of a SLI personto distinguish between the sounds. Additionally, for example, theseprior methods do not take into account the possible effects of thespectral content of the interfering sounds upon the ability of a SLIperson to differentiate between sounds presented in rapid sequence orsimultaneously. Thus, there has been a need to continue to developimproved methods for the identification and treatment of languageperception problems in specific language impaired (SLI) individualsbased upon other elements of sound patterns such as temporal ordering ofthe sounds and spectral content of the sounds. The present inventionmeets this need.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention provides a method, and apparatus for screeningindividuals for specific language impairment (SLI) and for trainingindividuals who suffer from SLI so as to remediate the effects of theimpairment. The novel method and apparatus takes into account both thespectral content of interfering sound stimuli and the temporal ordering(or direction) of the interference between the stimuli. It is well knownthat even persons with normal language ability may have difficultydetecting a target sound in the presence of masking noise with the samefrequency (or spectral) content as the target. Experiments have shown,however, that SLI persons often have greater difficulty than personswith normal language ability in detecting a target sound in the presenceof a masking noise with a different spectral content than the target.Moreover, experiments have shown that although the ability of SLIindividuals to detect a target sound in the presence of a masking noiseimproves as the spectral content of the target and the noise areseparated by increasing amounts, the ability of non-language impairedpersons to detect the target improves at a faster rate as the spectralseparation increases. Experiments also have shown that backward maskingoften is greater in individuals with SLI than in persons who do notexperience SLI. Thus, these experiments suggest that SLI persons havelower resolution frequency discrimination than non-SLI persons.Screening according to a presently preferred embodiment of the inventionassesses the severity of backward masking interference and the severityof spectral interference in simultaneous and non-simultaneous masking.Training according to the current embodiment aims to reduce the effectsof backward masking and spectral interference in persons with SLI.

Screening of an individual for SLI according to a current embodiment ofthe invention involves presenting the individual with several differentpatterns of target sound stimuli and mask sound stimuli. A targetstimulus is the target of reception. It is the sound signal component tobe identified by the individual under test. A target sound stimulus maybe a consonant, a tone, a tonal complex, noise, amplitude modulatednoise or a frequency sweep, for example. A mask sound stimulus is asound that may destructively interfere with the target sound stimulus soas to make the target more difficult to detect in the presence of themask. A mask sound stimulus may be noise with a prescribed spectralcontent, a tone, a vowel, a word, a word fragment, a sentence, asentence fragment the sound of many voices in a crowd, a tone, a tonalcomplex, noise, amplitude modulated noise or a frequency sweep, forexample. In the present implementation of the invention, many testpatterns comprising different combinations of target and mask soundstimuli are used to assess backward masking effects and spectralinterference effects in an individual under test.

Referring to the illustrative drawings of FIG. 1, there is shown anexemplary target-mask sound stimulus pattern which can be used to assessa person for impairment based upon backward masking effects. A series oftarget-mask stimuli pairs are provided in which individual targetstimuli and corresponding mask stimuli are temporally sequenced with thetarget preceding the corresponding mask. The exemplary pattern includesthe target-mask pair T4-M4, in which the target sound stimulus T4precedes the mask sound stimulus M4. The pattern also includestarget-mask pairs, T1-M1, T2-M2 and T3-M3. It will be appreciated thatonly a few representative target-mask pairs are shown, and that thereordinarily will be many more target-mask pairs in a typical pattern inaccordance with the invention. The time interval between target-maskpairs varies. For example, the time interval t4 between T4 and M4 isgreater than the time interval t3 in T3-M3 or the time interval t1 inT1-M1. Moreover, the time interval t3 is shorter than the time intervalst4, t2, or t1. In general, the shorter the duration of the target themore pronounced the masking effect. Target durations ranging from almost0 milliseconds (msc) to about 500 msc generally are employed in thescreening and training in accordance with the invention. An individualunder test is expected to indicate when he or she perceives a targetstimulus. An electronic record is created which records the individual'sresponses to the various target-mask pairs in the test pattern. Therecord which may be encoded in electronic media provides a map of theindividual's ability to discern target sound stimuli as a function ofthe temporal spacing between a target sound stimulus and a backward masksound stimulus.

Referring to the illustrative drawings of FIG. 2, there is shown anexemplary target-mask sound stimulus pattern that may be used to assessspectral masking effects according to the invention. A series oftarget-mask stimuli pairs are provided in which individual targetstimuli and corresponding mask stimuli occur simultaneously (or overlapin time) and in which the spectral content of the mask varies. Forexample, mask M10 lacks a frequency notch. In other words, mask M10 iscontinuous throughout the range of frequencies provided within M10.Consequently, mask M10 contains frequency components that overlap thefrequencies in the target T10. See, Patterson, Roy D., "Auditory FilterShapes Derived With Noise Stimuli", Journal of the Acoustical Society ofAmerica, Volume 59, No. 3, pp. 640-654. In contrast, mask M11 has aspectral notch with a width of delta F11; mask F12 has a spectral notchof delta F12; and mask has a frequency notch of delta F13. It will beappreciated from the illustrative drawings of FIG. 2 that the width ofthe spectral notch in M12 is greater than the widths of spectral notchesin M11 or M13, and that the width of the spectral notch in M11 isnarrower than the width of the spectral notch in M13. Ordinarily, thewider the frequency notch the easier it will be to detect the target inthe presence of the mask. In a presently preferred embodiment frequency(or spectral) notches of 0, 0.2/(target tone frequency), 0.4/(targettone frequency) and 0.8/(target tone frequency) are employed. Anindividual under test is expected to indicate when he or she perceives atarget stimulus in the presence of each of these separate masks. Morespecifically, the target sound stimulus intensity level at which theindividual detects the target T10 in the presence of mask M10 isdetermined. The target sound stimulus intensity level at which theindividual detects the target T11 in the presence of mask M11 isdetermined. The target sound stimulus intensity level at which theindividual detects the target T12 in the presence of mask M12 isdetermined. The target sound stimulus intensity level at which theindividual detects the target T13 in the presence of mask M13 isdetermined. See Rosen, Stuart and Baker, Richard J., "CharacterizingAuditory Filter Nonlinearity", Hearing Research, volume 73, 1994,pp.231-243. An electronic record is created which records theindividual's responses to the various target-mask pairs in the testpattern. The record which may be encoded in electronic media provides amap of the individual's ability to discern target sound stimuli as afunction of spectral interference between a target sound stimulus and amask sound stimulus.

The different target-mask sound stimulus patterns in FIGS. 1 and 2 areused to screen for two distinct problems in auditory perception.Target-mask patterns of the general type shown in FIG. 1 in which theinterstimulus interval is varied in a backward masking setting areparticularly helpful in evaluating the degree to which an individual isunable to detect a target sound stimulus due to backward maskingeffects. Usually, the greater the backward masking effect experienced bya person, the wider the time interval must be between target and maskbefore the person can detect the target in the presence of the followingmask. Alternatively, the greater the backward mask effect, the louderthe target stimulus must be in order to be detectable in the presence ofthe backward mask stimulus. Mask patterns of the general type shown inFIG. 2 in which the width of a spectral notch is varied are especiallyuseful in assessing the impact of spectral interference upon anindividual's ability to detect a target sound stimulus. Typically, thegreater the spectral interference experienced by a person, the wider thespectral notch must be between target and mask before the person candetect the target in the presence of the mask. Therefore, varying thespectrum of the mask can reveal the severity of both simultaneous(overlapping) and non-simultaneous (backward or forward) mask effects.Thus, the exemplary target-mask sound stimulus patterns of FIGS. 1 and 2are used to evaluate two independent auditory perceptual correlants.

There are significant advantages in screening individuals for SLI inaccordance with the present invention. For example, research shows thatSLI individuals often experience more severe backward masking effectsthan the normal (or non-SLI) population. Hence, the target-mask patternsexemplified in presence of a mask sound stimulus when the target andmask possess different spectral content. Thus, the target-mask patternsexemplified in FIG. 2 can be particularly useful in determining theextent of both simultaneous and non-simultaneous masking in SLI persons.

It will be appreciated that in backward masking, varying the spectralcontent of the mask can be used to ascertain the severity of thebackward mask problem. For instance, a series of target-mask pairs canbe provided in which the target is presented before the mask (backwardmask scenario); a spectal notch in the mask is varied throughout thepattern while ISI is fixed; and, for each different spectral notch, adetermination is made as to the threshold target stimulus level at whichan individual can detect the target. In this manner a map of anindividual's ability to detect a target in the presence of backwardmasks having different spectral content can be created.

Assessment of spectral interference in both non-simultaneous andsimultaneous masking are important to develop a more completeunderstanding of the basis for an SLI person's language impairment. Forexample, backward masking, a form of non-simultaneous masking, isbelieved to be an underlying cause of masking of a consonant sound by afollowing vowel sound. In the fragment /ba/, for instance, it isbelieved that the consonant /b/ in some cases may be backward masked bythe vowel /a/. In contrast, for example, simultaneous masking, in whichthe target and mask overlap in time, is believed to be an underlyingproblem in differentiating sounds that are similar or overlapping inspectral content. The short /i/ and the short /a/ sounds, for instance,sometimes may interfere with each other due to simultaneous maskingeffects. See Stark, Rachel E. and Heinz, John m., "Vowel Perception inChildren with and Without Language Impairment", Journal of Hearing andSpeech Research, Volume 39, pp.860-869. Of course, a betterunderstanding of the nature of a person's learning impairment may resultin the development of an improved and more realistic course of adaptivetraining for such individual.

EXAMPLE

The following example reports the results of psychophysical testsemploying simple tones and noises showing that children with specificlanguage impairment have severe auditory perceptual deficits for briefbut not long tones in particular sound contexts. The data supports theview that language difficulties result from problems in auditoryperception, and provide further information about the nature of theseperceptual problems.

FIG. 1: Average tone level required by 8 language-impaired (filledsquares) and 8 control (open squares) children to just detect a longtone temporally centered in a bandpass noise (panel A), or a brief tonepresented before, during or after that noise (panel B). The error barsindicate plus and minus one standard error of the mean across subjects.The stimuli are illustrated schematically along the abscissa.

FIG. 2: Average tone level required by 8 language-impaired (panel A) and8 control (panel B) children to just detect a brief tone presentedbefore, during or after a bandpass (squares, replotted from FIG. 1B) ornotched (triangles) noise. The error bars indicate plus and minus onestandard error of the mean across subjects. The stimuli are illustratedschematically at the bottom of the figure.

We measured the detection threshold for a brief tone presented before,during or after two different masking noises in 8 children diagnosedwith specific language impairment, and in 8 control children with normallanguage skills who matched the others in age and nonverbalintelligence. Details of the two groups are provided in Table 1. Beforebeginning the tests with the brief tones, we introduced the children tothe listening task by measuring their detection thresholds for a longtone presented in the temporal center of a "bandpass" noise thatincluded frequencies at and near the tone frequency. The points abovethe schematic illustration of the stimuli in FIG. 1A show that the samemean tone level was required by specifically language impaired (filledsquares) and control (open squares) children to detect the long tone inthis masking condition [F(1,14)<1p>.05].

FIG. 1B shows the results of our subsequent measurements in the samechildren of the detection threshold for a brief tone presented with thebandpass noise at each of four temporal positions illustratedschematically along the abscissa. The performance pattern of controlchildren (open squares) was just as expected based on previous work onnormal auditory masking: The tone was easier to detect when it waspresented just before or just after, as opposed to during, the noise,and was easiest to detect when it preceded rather then followed thenoise. In comparison to controls, children with specific languageimpairment (filled squares) needed a higher tone level for detection inevery condition. Most remarkably, impaired children had as much or moredifficulty detecting the tone when it was presented before the noise(the backward-masking condition) as when it was presented during orafter the noise. There was no overlap in performance between the twogroups in the backward-masking condition.

We also measured detection thresholds in the same children for a brieftone presented at each of the four temporal positions in a spectrally"notched" noise that excluded frequencies at and near the tonefrequency. The two panels of FIG. 2 show the mean tone thresholds foreach group for both the bandpass (squares; replotted from FIG. 1B) andnotched (triangles) noises. The conditions are schematically illustratedat the bottom of the figure. For both impaired and control children, thetone was typically easier to detect with the notched than with thebandpass noise. This is expected in normal hearing. However, in contrastto most adults, neither group of children showed a clear thresholddifference between the two masker types when the tone and noise startedsimultaneously.

Two aspects of the performance of impaired children with the notchednoise are particularly important. First, language-impaired children werebetter at hearing the tone presented before (the backward-maskingcondition) the notched than the bandpass noise. Their severe perceptualdeficit for tones presented in this temporal position was worst when thetone and following noise had similar frequencies. Follow-up tests on 4additional impaired children showed that the detection threshold in thebackward masking condition reached control values when the spectralnotch in the masker was made sufficiently wide. Thus, impaired childrenhad perceptual difficulties in certain temporal and spectral soundcontexts, but did not display a general deficit in the perception ofrapidly presented sounds. Second, the mean threshold difference betweenthe notched and bandpass noises was smaller for impaired than forcontrol children in both the simultaneous-delay (10.5 dB vs. 18.6 dB)and forward (15.7 dB vs. 20.5 dB) masking conditions. This indicatesthat impaired children were less able than controls to take advantage ofa frequency separation between the tone and noise to aid detection ofthe tone.

A 2×2×4 analysis of variance performed on the data in FIG. 2 revealedsignificant main effects for subject group [F(1, 112)

=

102.70, p<0.0001], noise type [F(1, 112)

=

43.94, p<0.0001], and tone position [F(3, 112)

=

41.93, p<0.0001]. There were also significant interactions betweensubject group and tone position [F(3, 112)

=

16.72, p<0.0001] and noise type and tone position [F(3, 112)

=

3.53, p

=

017], but not between the subject group and the noise type [F(1, 112)

=

0.08, p>0.05], nor among the three factors [F(3, 112)

=1.07, p>0.05]. A Scheffe post hoc analysis indicated that thethresholds of the two subject groups differed significantly only in thetwo backward-masking conditions (p<0.0001 for the bandpass masker and p

=

0.0002 for the notched-noise masker).

In sum, these results suggest that children with specific languageimpairment are severely impaired in their ability to (1) separate abrief sound from a rapidly following sound of similar frequency, and (2)enhance the detection of a brief tone by exploiting a frequencydifference between the tone and a longer co-occurring or precedingmasking sound. These auditory perceptual deficits could clearly degradethe perception of the brief acoustic elements of speech. Manyindividuals with language impairment and other disorders related tospoken language might benefit from diagnoses incorporating the auditorytests used here, and from auditory training that focuses on their mostseverely impaired abilities. The present auditory tests might also aidin the diagnosis and treatment of persons with reading difficulties. Wehave preliminary data on 12 such individuals. Five had excessive amountsof auditory backward masking, but none had as much masking as thechildren with specific language impairment. Our results are in accordwith the conclusion of a recent review that some, but not all, childrenwith reading problems have difficulties accurately perceiving rapidlypresented stimuli. Our data are also consistent with a previous reportshowing that children with reading difficulties are particularly poor atdiscriminating words that differ only in their first sound. Finally, thetemporal and spectral specificity of the auditory perceptual deficitsreported here may serve to guide the search for the underlying neuralbases of language disorders.

METHODS

Stimuli: All stimuli were generated digitally. Tone: 1000 Hz, 20 or 200ms onset-to-offset. Noises: 600-1400 Hz (bandpass noise) or 400-800 Hzand 1200-1600 Hz (notched noise), 300 ms onset-to-offset, 40 dB SPLspectrum level. Gating envelope: 10-ms cosine squared for all stimuli.Masking conditions: The 20-ms tone was turned on at four different timesdefined relative to the onset of the 300-ms noise: -20 ms (backwardmasking), 0 ms (simultaneous-onset masking), 200 ms (simultaneous-delaymasking), or 300 ms (forward masking). The 200-ms tone was turned on 50ms after noise onset.

Procedure: We used a standard, adaptive, two-interval forced-choiceprocedure to estimate the tone level required for 94% correctdetections. The observation intervals were separated by 800 ms. Visualdisplays on a computer screen marked the observation intervals and gavefeedback. Each reported brief-tone threshold was based on the mean oftwo or three 30-trial measurements per child. Three measurements werealways collected, but the most deviant estimate was omitted if thestandard deviation of the three was greater than 15 dB. The averagewithin-subject standard error was 3.7 dB for impaired children and 2.5dB for control children. Because the long-tone condition was used toacquaint the children with the task, we report only the last thresholdestimate of the one to three obtained from each child in that condition;in total, the 8 impaired children completed 14 threshold estimates andthe 8 control children 13 estimates in the long-tone condition. Wecollected the data with the long tone first, and then presented the fourbrief-tone conditions in pairs [bandpass then notched noise] in adiagram balanced latin square. We tested each child individually in asound-attenuated room. Stimuli were delivered to the right ear overSennheiser HD450 headphones. All children were paid for theirparticipation.

                  TABLE 1                                                         ______________________________________                                                     Language Impaired                                                             MEAN (sd)             Control                                    ______________________________________                                        Age            8.1(6.3)       8.0(7.1)                                        Sex                                            3 M, 5 F                       TONI-2                                         105.1(6.5)                     CELF-R:                                                                       Receptive Language                                                                               83.4(6.9).sup.+                                                                                  112.5(7.8)                              Linguistic Concepts                                                                             7.4(1.5).sup.+                                                                                     10.6(2.3)                              Sentence Structure                                                                               6.9(1.6)                    12.4(2.0)                      Ora1 Directions                                                                                     7.5(2.1)                                                                                               13.0(2.1)                      Expressive Language                                                                             70.7(4.8).sup.+                                                                                   98.1(5.2)                               Word Structure         5.5(1.6)                                                                                              8.8(1.6)                       Formulated Sentences                                                                           4.9(1.3)                      8.6(1.1)                       Recalling Sentences                                                                             6.3(2.1)                     12.0(1.3)                      Total Language         75.3(4.5).sup.+                                                                              105.3(5.4)                              ______________________________________                                    

Apparatus Set Up and Screening Maps

Screening and training in accordance with the present invention can beperformed using a standard personal computer equipped with an inputdevice such as a keyboard or a mouse and headphones to deliver patternof target-mask pairs to an individual. Conventional signal generationequipment can be used to actually generate the target-mask patterns. Theillustrative drawing of FIGS. 3 and 3A-3C describe the flow of acomputer program that can control the generation and presentation of thetarget-mask stimulus patterns. The illustrative drawings of FIGS. 4A-4Care two dimensional mappings of three different individuals tested inaccordance with the invention for the ability to detect a target in abackward masking situation with a 20 msc target length and without aspectal notch in the mask. The filled circles represent data points(mask level and ISI) for which the person under test could detect thetarget. At a tone level of 0, the target has the same intensity as themask. The map of FIG. 4A represents result for a normal, non-languageimpaired person. The map of FIG. 4B represents results for a mildlylanguage impaired person. The map of FIG. 4C represents results for aseverely language impaired person.

We claim:
 1. A method of screening individuals for audio perceptionproblems, the method comprising the steps of:a) selecting a target soundstimulus and a mask sound stimulus, the starting level of the targetsound stimulus being sufficiently large relative to the starting levelof the mask sound stimulus so that individuals with normal audioperception can detect the target sound stimulus in the presence of themask sound stimulus when the target sound stimulus is separated from themask sound stimulus by at least a prescribed interstimulus interval(ISI); b) changing the target sound stimulus level relative to the masksound stimulus level by a prescribed amount; c) producing the targetsound stimulus and the mask sound stimulus with the changed target soundstimulus level while maintaining the prescribed ISI; d) determiningwhether the individual can detect the target sound stimulus; and e) ifthe individual cannot detect the target sound stimulus signal thenrepeating the above steps until the individual can detect the targetsound stimulus.
 2. The method of claim 1 wherein said step of changingthe target sound stimulus level relative to the mask sound stimuluslevel involves changing the level of the target sound stimulus whilemaintaining the mask sound stimulus constant.
 3. The method of claim 1wherein said step of changing the target sound stimulus level relativeto the mask sound stimulus level involves increasing the level of thetarget sound stimulus while maintaining the mask sound stimulusconstant.
 4. The method of claim 1 wherein said step of changing thetarget sound stimulus level relative to the mask sound stimulus levelinvolve decreasing the level of the mask sound stimulus.
 5. A method ofscreening individuals for audio perception problems, the methodcomprising the steps of:a) selecting a target sound stimulus and a masksound stimulus, the starting level of the target sound stimulus beingsufficiently large relative to the starting level of the mask soundstimulus so that individuals with normal audio perception can detect thetarget sound stimulus in the presence of the mask sound stimulus whenthe target sound stimulus is separated from the mask sound stimulus byat least a prescribed interstimulus interval (ISI); b) changing thetarget sound stimulus level relative to the mask sound stimulus level bya prescribed amount; c) producing the target sound stimulus and the masksound stimulus with the changed target sound stimulus level whilemaintaining the prescribed ISI; d) determining whether the individualcan detect the target sound stimulus; and e) if the individual candetect the target sound stimulus signal then repeating the above stepsuntil the individual cannot detect the target sound stimulus.
 6. Themethod of claim 5 wherein said step of changing the target soundstimulus level relative to the mask sound stimulus level involveschanging the level of the target sound stimulus while maintaining themask sound stimulus constant.
 7. The method of claim 5 wherein said stepof changing the target sound stimulus level relative to the mask soundstimulus level involves decreasing the level of the target soundstimulus while maintaining the mask sound stimulus constant.
 8. Themethod of claim 5 wherein said step of changing the target soundstimulus level relative to the mask sound stimulus level involveincreasing the level of the mask sound stimulus.